Regex (Regular Expression)

A regex (regular expression) is a sequence of characters that defines a search pattern. You use it to find, validate, extract or replace text, for example to check that a string looks like an email address or to pull status codes out of a log file.

Why it matters

Regex turns many lines of string-handling code into one pattern and is built into most languages, editors and command-line tools such as grep. Syntax details vary slightly between engines, so a pattern should be tested in the engine you will actually use.

Example

Pattern:  ^(\d{3}) (GET|POST) (\S+)$
Text:     404 GET /missing-page
Groups:   1 = 404, 2 = GET, 3 = /missing-page

\d{3} matches exactly three digits, (GET|POST) matches either word, \S+ matches non-space characters, and ^ and $ anchor the start and end of the line.

Common mistakes

• Forgetting to escape special characters such as ., which matches any character, not a literal dot.
• Writing nested quantifiers like (a+)+ that cause catastrophic backtracking and freeze the program on certain input.
• Using regex to fully validate emails or parse HTML, where a dedicated parser or library is safer.

Related

• Function
• Variable
• Debugging
• Tool: Regex tester

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